Solution for removing aluminum oxide film and method for surface treatment of aluminum or aluminum alloy

ABSTRACT

A solution for removing an aluminum oxide film from an aluminum or aluminum alloy surface, which includes a salt or oxide of a metal capable of substituting aluminum, a solubilizing agent for ions of the metal, and an alkali, and which has a pH Of 10 to 13.5. 
     The removing solution makes it possible to form a film of the metal derived from the metal salt or oxide contained in the removing solution by dissolving away the oxide film from the aluminum or aluminum alloy surface at a low temperature and a high speed while restraining, as securely as possible, erosion of the aluminum or aluminum alloy surface. Besides, the removing solution ensures that even in the case where the thickness of the aluminum or aluminum alloy basis material is very small, the aluminum or aluminum alloy surface can be activated while assuredly leaving the aluminum or aluminum alloy basis material.

BACKGROUND OF THE INVENTION

The present invention relates to a solution for removing an aluminumoxide film and a surface treatment method for aluminum or an aluminumalloy, particularly to a solution for removing an aluminum oxide filmand a surface treatment method for aluminum or an aluminum alloy whichare effective when used in or as a pre-treatment in forming a UBM(under-bump metal) or bumps on a wafer by plating.

As a method for forming a UBM or bumps on a silicon wafer, there hasbeen used a method including the steps of subjecting aluminum thin filmelectrodes patterned on a wafer to a zinc substitution treatment so asto form a zinc film and then performing electroless plating to formbumps, a method including the steps of conducting a palladium treatmentin place of the just-mentioned zinc substitution treatment and thenperforming electroless plating to form bumps, or a method including thesteps of directly replacing the surfaces of aluminum thin filmelectrodes with nickel and then performing autocatalytic electrolessplating to form bumps.

Here, irrespectively of the method selectively used to form the UBM orbumps, a degreasing treatment of the aluminum thin film electrodes, atreatment for removing an aluminum oxide film, metallic impurities orthe like present on the aluminum thin film electrodes or the liketreatment is ordinarily conducted as a pretreatment. In this case, analuminum oxide film produced in an extremely small thickness uponimmersion in nitric acid or the like can be subjected directly to aplating treatment in the subsequent step to thereby achieve the desiredplating without any problem. On the other hand, in the case where arigid aluminum oxide film generated through such a manufacturing step asa grinding step and an annealing step is left on the surface of theworkpiece, the plating film formed in the subsequent step may showinsufficient adhesion or pits may be formed in the plating film, and, inworse conditions, the desired plating film may not be depositedsuccessfully. Therefore, it is desirable to remove such a rigid aluminumoxide film completely before plating.

To cope with such a problem, a method in which a substrate for platingis formed by a dry process without conducting dissolution of thealuminum oxide film has been proposed (refer to Japanese PatentLaid-open No. Hei 11-87392). However, the method has room forimprovement in that it involves complicated steps and it isdisadvantageous from the viewpoints of speediness and production cost.Furthermore, the method has the problem that the non-conductive propertyof the remaining oxide film leads to an increased thermal resistance,resulting in worsening of electrical properties.

Removal of the rigid aluminum oxide film has been carried out by a wetmethod. Specifically, the oxide film has been removed by a method inwhich the workpiece is immersed in a strong alkaline solution or acidicsolution so as to completely remove the oxide film while partlydissolving the underlying aluminum or aluminum alloy basis material.While this method may be satisfactory where the aluminum or aluminumalloy basis material is thick, it becomes very difficult to secure amargin of etching when the thickness of the aluminum or aluminum alloybasis material is reduced to 0.5 μm or 1.0 μm.

In addition, a method in which an organic solvent is used (refer toJapanese Patent Laid-open No. 2002-151537), a method in which a mixtureof several acids is used (refer to Japanese Patent Laid-open No. Hei5-65657 and JP-A-2002-514683) and the like have also been proposed.

However, in these methods, considerable etching of the aluminum oraluminum alloy basis material cannot be obviated, and, where the basismaterial is a thin film, the thin film would be lost or dissolved;therefore, it is difficult to select appropriate treatment conditions.Furthermore, a grinding or other mechanical polishing step cannot beadopted for the thin film, unlike the case of die casting. Therefore,the oxide film formed through a heat treatment in the manufacturingprocess would be left on the surface of the aluminum thin film, whichworsens the situation.

Incidentally, Japanese Patent Laid-open No. 2004-263267 is alsomentioned as a related-art document.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of theabove-mentioned circumstances. Accordingly, it is an object of theinvention to provide a removing solution for an aluminum oxide film anda surface treatment method for aluminum or an aluminum alloy using theremoving solution by which it is possible to remove a rigid oxide filmeasily, speedily and assuredly without excessively etching an aluminumor aluminum alloy surface and to effectively treat even a workpieceprovided with an aluminum or aluminum alloy thin film on the surfacethereof.

The present inventors made intensive and extensive investigations forattaining the above object. As a result of the investigations, it wasfound out that when an aluminum oxide film formed on aluminum or analuminum alloy is treated with a removing solution which including ametallic salt or oxide containing a metal capable of substitutingaluminum, a solubilizing agent for ions of the metal, an alkali and,optionally, a surfactant and which has a pH of 10 to 13.5, the oxidefilm can be speedily removed at a low temperature while restraining, assecurely as possible, the erosion of the aluminum or aluminum alloysurface.

Specifically, the cause of the considerable erosion of an aluminum oraluminum alloy basis material which would occur at the time of removingan aluminum oxide film by use of a prior art removing solutioncontaining an acid as a main component lies in that it would beimpossible to effectively cope with the difference between thereactivity between the aluminum oxide film and the acid and thereactivity between the aluminum or aluminum alloy basis material and theacid.

FIG. 2 shows schematic sectional views showing the manner in which analuminum or aluminum alloy surface is activated by use of a prior arttreating solution containing an acid as a main component, wherein FIGS.2A to 2D respectively show stages of a process of removing the oxidefilm from the aluminum or aluminum alloy surface by use of a prior arttreating solution.

Ordinarily, the thickness of the aluminum oxide film 2 is not uniform,and the acidic treating solution 31 for removing the aluminum oxide film2 does not behave uniformly. Therefore, as dissolution of the aluminumoxide film 2 proceeds, the base material of the aluminum or aluminumalloy 1 is first exposed partially (FIG. 2A).

Here, since the reactivity between the aluminum or aluminum alloy 1 andthe acid (acidic treating solution) 31 is higher than the reactivitybetween the aluminum oxide film 2 and the acid 31, a reaction asrepresented by the following formula (1) proceeds preferentially, sothat the aluminum or aluminum alloy 1 is dissolved preferentially (FIG.2B). In this instance, the pH in the vicinity of reaction points isrelatively raised due to consumption of hydrogen ions, and theconcentration of aluminum(III) hydroxide is raised due to a reaction asrepresented by the following formula (2). The rise in the concentrationof aluminum(III) hydroxide leads to further suppression of thedissolution of the aluminum oxide film 2.

2Al+6H⁺→2Al³⁺+3H₂  (1)

Al³⁺+3OH⁻→Al(OH)₃  (2)

As the workpiece is immersed in the treating solution further, thedissolution of the aluminum or aluminum alloy 1 proceeds further,resulting in that the aluminum or aluminum alloy 1 underlying thealuminum oxide film 2 is dissolved away (FIG. 2C). Eventually, thealuminum oxide film 2 is physically delaminated, resulting in that thealuminum oxide film having been present on the surface of the aluminumor aluminum alloy basis material is removed (FIG. 2D).

In the case where the prior art treating solution containing an acid asa main component is used, the aluminum oxide film is removed through thejust-mentioned stages, so that the considerable erosion of the aluminumor aluminum alloy basis material is unavoidable. Therefore, this methodhas been utterly inapplicable to the case where the workpiece to bedeprived of an aluminum oxide film is a thin layer, such as an aluminumthin film electrode on a silicon wafer.

On the other hand, as a result of the present inventors' investigationsin seeking a method for dissolving away an aluminum oxide film whileobviating a high reactivity between an aluminum or aluminum alloy basismaterial and an acid, they found out that an alkaline (basic) removingsolution with a salt or oxide of a metal capable of substitutingaluminum added thereto together with a solubilizing agent for the ionsof the metal is effective, as above-mentioned.

More specifically, FIG. 1 shows schematic sectional views showing themanner in which an aluminum oxide film present on an aluminum oraluminum alloy surface is removed by use of a removing solution based onthe present invention, wherein FIG. 1A is a stage in which the aluminumor aluminum alloy basis material is partially exposed, like in the priorart example shown in FIG. 2A.

Here, a metallic film 4 derived from the salt or oxide of that metalcapable of substituting aluminum which is contained in the removingsolution (alkaline removing solution) 3 based on the present inventionis swiftly formed in the areas where the aluminum or aluminum alloy 1 isexposed (FIG. 1B).

Since aluminum in the aluminum oxide film 2 has already been ionized,the metallic film derived from the metallic salt or oxide would not beformed on the aluminum oxide film 2. In addition, the aluminum oraluminum alloy 1 would not be eroded, since it is protected by thatmetallic film 4 derived from the salt or oxide of the metal capable ofsubstituting aluminum which has been formed on the exposed portions ofthe aluminum or aluminum alloy 1. Therefore, as this reaction continues,the metallic film 4 derived from the salt or oxide of the metal capableof substituting aluminum is sequentially formed on those portions of thealuminum or aluminum-alloy 1 which are exposed attendant on the progressof the dissolution of the aluminum oxide film 2 (FIG. 1C). Eventually,the aluminum oxide film 2 having been present on the surface of thealuminum or aluminum alloy 1 is completely dissolved away. On the otherhand, the aluminum or aluminum alloy surface is wholly covered with themetallic film 4 derived from the salt or oxide of the metal capable ofsubstituting aluminum (FIG. 1D).

In short, as shown in FIG. 1, when the removing solution based on thepresent invention is used, the aluminum or aluminum alloy basis materialexposed by etching is immediately coated with the substituent metallayer, so that erosion of the aluminum or aluminum alloy basis materialis restrained. In addition, the dissolution of the aluminum oxide filmwould not be suppressed due to an increase in the concentration ofaluminum hydroxide attendant on dissolution of the aluminum or aluminumalloy basis material, so that effective removal of the aluminum oxidefilm will proceed continuedly.

Besides, with the removing solution made to be alkaline (acidic), thenumber of hydroxide (OH⁻) ions is increased, and, since the hydroxide(OH⁻) ions have a strong behavior of dissolving the aluminum oxide film,the intended treatment can be carried out at a lower temperature and ina shorter time, as compared with the case of using an acidic treatingsolution. Further, though the reactivity between the removing solutionand the metallic aluminum of the basis material is also high where theremoving solution is alkaline (basic), it was found out that thisproblem can be solved by suppressing the alkalinity (pH), which is anindex of the concentration of free alkali (base). Based on the finding,the present invention has been completed.

Therefore, the removing solution containing the metal capable ofsubstituting aluminum based on the present invention is a removingsolution with which it is possible to dissolve away an aluminum oxidefilm continuedly at a low temperature and speedily, while minimizingerosion of an aluminum or aluminum alloy basis material, through a wetprocess.

Accordingly, the present invention provides the following solutions forremoving an aluminum oxide film and the following methods for surfacetreatment of aluminum or an aluminum alloy.

(1): A solution for removing an aluminum oxide film from an aluminum oraluminum alloy surface, comprising a salt or oxide of a metal capable ofsubstituting aluminum, a solubilizing agent for ions of the metal, andan alkali and having a pH of 10 to 13.5.(2): The solution for removing the aluminum oxide film of (1), furthercomprising a surfactant.(3): A method for surface treatment of aluminum or the aluminum alloy,comprising the steps of:

immersing a workpiece having aluminum or the aluminum alloy at least atthe surface in the solution of (1), and

forming a substituent metal layer of the metal capable of substitutingaluminum and contained in the solution on the aluminum or aluminum alloysurface while removing the aluminum oxide film from the aluminum or thealuminum alloy surface.

(4): The method for surface treatment of aluminum or the aluminum alloyof (3), wherein the workpiece is a body having the aluminum or aluminumalloy film formed on a surface of a non-aluminum material.(5): The method for surface treatment of aluminum or the aluminum alloyof (3), wherein after the substituent metal layer is formed, a platinglayer is formed.(6): The method for surface treatment of aluminum or the aluminum alloyof (3), wherein after the substituent metal layer is formed, thesubstituent metal layer is removed by an acidic liquid having anoxidizing behavior.(7): The method for surface treatment of aluminum or the aluminum alloyof (6), wherein after the substituent metal layer is removed by theacidic liquid having the oxidizing behavior, the aluminum or aluminumalloy is subjected to a zinc substitution treatment or a palladiumtreatment and is then plated.(8): The method for surface treatment of aluminum or the aluminum alloyof (6), wherein after the substituent metal layer is removed by theacidic liquid having the oxidizing behavior, the aluminum or aluminumalloy is subjected directly to plating.

The removing solution based on the present invention makes it possibleto form a metallic film, derived from the salt or oxide of the metalcontained in the removing solution, on an aluminum or aluminum alloysurface while restraining as securely as possible the erosion of thealuminum or aluminum alloy surface, and the metallic film thus formedcan be dissolved away speedily at a low temperature withoutsubstantially eroding the aluminum or aluminum alloy surface. Therefore,by use of the removing method, the aluminum or aluminum alloy surfacecan be activated while securely leaving the aluminum or aluminum alloybasis material even where the thickness of the aluminum or aluminumalloy basis material is very small. The surface treatment method basedon the present invention can be preferably used in the case of, forexample, an activating treatment for the surfaces of aluminum thin filmelectrodes formed on a silicon wafer, particularly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are schematic sectional views sequentially showing themanner in which an aluminum oxide film is removed from an aluminum oraluminum alloy surface by use of a removing solution based on thepresent invention.

FIGS. 2A to 2D are schematic sectional views sequentially showing themanner in which an aluminum oxide film is removed from an aluminum oraluminum alloy surface by use of a removing solution according to aprior art.

DETAILED DESCRIPTION OF THE INVENTION

Now, the present invention will be described more in detail below.

The removing solution for an aluminum oxide film based on the presentinvention contains a salt or oxide of a metal capable of substitutingaluminum, a solubilizing agent for ions of the metal, and an alkali, andhas a pH of 10 to 13.5.

The metal constituting the metal salt or metal oxide contained in theremoving solution based on the present invention is not particularlylimited insofar as it is a metal capable of substituting aluminum. It ispreferable that the metal is a metal lower than aluminum in ionizationtendency, such as manganese, zinc, iron, cobalt, nickel, tin, lead,copper, mercury, silver, platinum, gold, and palladium. Examples of themetal salt include water-soluble salts, such as nitrates and sulfates,of the metal as just-mentioned. Among these metals, manganese and zincare preferred because they little differ in oxidation-reductionpotential from aluminum serving as the basis material.

The concentration of the metal salt or metal oxide used in the removingsolution based on the present invention is not particularly limited, andis normally not less than 1 ppm (mg/L), preferably, not less than 10 ppm(mg/L) and normally not more than 10,000 ppm (mg/L), preferably, notmore than 5,000 ppm (mg/L). If the concentration of the metal salt ormetal oxide is too low, the metal may not sufficiently substitutealuminum present as the basis material, or it may become necessary toreplenish the metal salt or metal oxide. On the other hand, if theconcentration is too high, in the case where the aluminum or aluminumalloy basis material is electrodes patterned on a wafer, the removingsolution may erode other members than the aluminum or aluminum alloybasis material, or the metal may be deposited even on other members thanthe aluminum or aluminum alloy basis material.

The solubilizing agent for the metal ions contained in the removingsolution based on the present invention is not particularly limited;normally, complexing agents and chelating agents can be used. Specificexamples of the agents that can be used include hydroxycarboxylic acidssuch as glycolic acid, lactic acid, malic acid, tartaric acid, citricacid, gluconic acid, heptogluconic acid, etc. and their salts;aminocarboxylic acids such as glycine, aminodicarboxylic acid,nitrilotriacetic acid, EDTA, hydroxyethylethylenediaminetriacetic acid,diethylenetriaminepentaacetic acid, polyaminopolycarboxylic acid, etc.and their salts; phosphorous acid chelating agents such as HEDP,aminotrimethylphosphonic acid, ethylenediaminetetramethylphosphonicacid, etc. and their salts; and amine chelating agents such asethylenediamine, diethylenetriamine, triethylenetetramine, etc.

The concentration of the solubilizing agent used in the removingsolution based on the present invention is not particularly limited, andthe total concentration of the solubilizing agent(s) is preferably 0.5to 10 times (molar ratio), more preferably 0.8 to 5 times (molar ratio)that of the metal salt used.

The alkali contained in the removing solution based on the presentinvention is not particularly limited, but the alkali has to be analkali (base) which will dissolve the oxide film. Examples of the alkaliwhich can be used include alkali metal hydroxides such as LiOH, NaOH,KOH, etc. and quaternary ammonium hydroxides such as tetramethylammoniumhydroxie (TMAH), choline, etc. Incidentally, the amount of the alkaliadded is such an amount as to bring the pH of the removing solution intothe specified range of 10 to 13.5, preferably, 11 to 13. If the pH isless than 10, the rate of dissolution is lowered markedly, whereas ifthe pH exceeds 13.5, the dissolution rate becomes too high to control.

The oxide film removing solution for aluminum or an aluminum alloy basedon the present invention, preferably, contains a surfactant, from theviewpoint of imparting water wettability. The surfactant used here isnot particularly limited. Examples of the surfactant which can be usedinclude nonionic surfactants such as polyethylene glycol,polyoxyethylene-oxypropylene block copolymer surfactant, etc. and,further, anionic and cationic surfactants. Among these surfactants,nonionic and anionic surfactants are preferred from the viewpoint ofuniform treatability. These surfactants may be used either singly or incombination of two or more of them.

For example, where polyethylene glycol is used as the surfactant, itsmolecular weight is not particularly limited, and is normally not lessthan 100, preferably not less than 200 and normally not more than20,000, preferably not more than 6,000. If the molecular weight is toohigh, the polyethylene glycol may be poor in solubility, whereas if themolecular weight is too low, water wettability may not be obtainedsuccessfully. Incidentally, as the polyethylene glycol, those which arecommercially available can be used.

Besides, the concentration of the surfactant in the removing solution isnot particularly limited. The concentration is normally not less than 1ppm (mg/L), preferably not less than 10 ppm (mg/L) and is normally notmore than 5,000 ppm (mg/L), preferably not more than 2,000 ppm (mg/L).If the concentration of the surfactant in the removing solution is toolow, the effect on water wettability obtained by the addition of thesurfactant may be low. On the other hand, if the concentration is toohigh, the substituent metal may be deposited on other members than thealuminum or aluminum alloy basis material.

Incidentally, the removing solution based on the present invention ispreferably prepared as an aqueous solution, from the viewpoint of safetyin handling. However, other solvents such as methanol, ethanol, IPA,etc. may be used, either as they are or in mixture with water.Incidentally, these solvents may be used either singly or in combinationof two or more of them.

The method of surface treating a workpiece by use of the removingsolution as above-described is conducted as follows. A workpieceprovided with aluminum or an aluminum alloy at least at a surfacethereof is immersed in the removing solution, whereby a metallic film(substituent metal layer) of the metal derived from the metal salt ormetal oxide contained in the removing solution is formed on the aluminumor aluminum alloy surface of the workpiece. In this case, after themetallic film is formed, the metallic film can be removed by use of anacidic liquid having an oxidizing behavior. Besides, the metallic filmor the aluminum or aluminum alloy deprived of the metallic film may besubjected directly to plating or subjected to plating after undergoing azinc substitution treatment or a palladium treatment.

The immersion conditions in immersing the workpiece having aluminum oran aluminum alloy in the removing solution are not particularly limited,and can be appropriately set taking the thickness of the aluminum oxidefilm to be removed, etc. into account. The immersion time is normallynot less than 1 minute, preferably not less than 2 minutes and isnormally not more than 20 minutes, preferably not more than 15 minutes.If the immersion time is too short, the substitution may not proceedsufficiently and the oxide film may be removed insufficiently. On theother hand, if the immersion time is too long, the removing solution maypenetrate through small holes in the substituent metal layer, possiblyleading to elution of aluminum or the aluminum alloy.

In addition, the temperature at the time of immersion is notparticularly limited. The immersion temperature is normally not lessthan 20° C., preferably not less than 25° C. and is normally not morethan 100° C., preferably not more than 95° C. If the immersiontemperature is too low, dissolution of the oxide film may not beachieved. On the other hand, if the immersion temperature is too high,other members than the aluminum or aluminum alloy basis material may beeroded. Incidentally, the immersion is preferably attended by agitationof the solution or oscillation of the workpiece, from the viewpoint ofachieving a uniform treatment.

The workpiece having aluminum or an aluminum alloy at least at a surfacethereof which is under consideration in the present invention may be aworkpiece formed entirely of aluminum or the aluminum alloy, or may be aworkpiece in which the surface of a non-aluminum member (for example,silicon or FRA (substrate of printed wiring board)) is wholly or partlycoated with aluminum or the aluminum alloy. In addition, the form ofaluminum or the aluminum alloy is also not particularly limited.Favorable examples of the applicable form include a blank material, arolled material, a cast material, a coating film, etc. Incidentally, inthe case where an aluminum or aluminum alloy coating film is formed on asurface of a non-aluminum member, the coating film forming method is notparticularly limited. Favorable examples of the coating film formingmethod include gaseous phase plating methods such as vacuum evaporation,sputtering, ion plating, etc.

The thickness of the coating film is normally not less than 0.5 μm,preferably not less than 1 μm, from the viewpoint of securely leavingthe aluminum or aluminum alloy basis material in using the surfacetreatment method based on the present invention. Incidentally, there isno particular upper limit to the thickness, and the thickness isnormally up to 100 μm. It should be noted here that since the removingsolution based on the present invention will scarcely erode the aluminumor aluminum alloy basis material, the removing solution can effectivelybe used particularly for coating films with a thickness of not more than1.0 μm, to which the treating solutions in a prior art could not beapplied because of the problem that the basis material would be too thinafter the treatment.

Furthermore, the constituent of the coating film is not particularlylimited insofar as it is aluminum or an aluminum alloy. For example, thesurface treatment method based on the present invention can be favorablyapplied to coating films of Al—Si (Si content: 0.5 to 1.0 wt %), Al—Cu(Cu content: 0.5 to 1.0 wt %), etc.

After the above-mentioned workpiece is immersed in the removing solutionbased on the present invention, the metallic film formed on the surfaceof the workpiece (the layer of the substituent metal derived from thesalt or oxide of that metal capable of substituting aluminum which iscontained in the removing solution based on the present invention) ispreferably removed before a post-treatment, in the case where theadhesion of the film may possibly lowered at the time of the subsequenttreatment or in the case where such a metallic layer is inherentlyunnecessary.

In dissolving the metallic film, an acidic liquid having an oxidizingbehavior is used, from the viewpoint of moderating the reactivity withaluminum or the aluminum alloy present as the basis material. In thiscase, preferable examples of the acidic liquid having an oxidizingbehavior include acids having an oxidizing behavior such as nitric acid,etc. and aqueous solutions thereof; and solutions obtained by adding atleast one oxidizing agent, such as hydrogen peroxide, sodium persulfate,ammonium persulfate, potassium persulfate, etc. to an acid not having anoxidizing behavior, such as sulfuric acid, hydrochloric acid, etc. Inthis case, the acid has the behavior of dissolving the substituentmetal, while the oxidizing agent has the behavior of moderating thereactivity with the aluminum or aluminum alloy basis material.Incidentally, among the oxidizing agents, hydrogen peroxide is preferredbecause it consists of hydrogen and oxygen and it is converted to waterwhen reduced. Besides, sodium persulfate and potassium persulfate arepreferred, from the viewpoints of stability and easiness to handle.

Where nitric acid is used as the acid (and oxidizing agent), the amountof nitric acid in the dissolving liquid (aqueous solution) is normallynot less than 200 ml/L, preferably not less than 300 ml/L and isnormally not more than 1,000 ml/L, preferably not more than 700 ml/L. Ifthe amount of nitric acid is too small, the oxidizing power is so lowthat the reaction may not be stopped. Incidentally, the nitric acidamount of 1,000 ml/L refers to the case where the solution is whollycomposed of nitric acid.

Besides, the amount of the oxidizing agent in the dissolving liquid inthe case of using the oxidizing agent is normally not less than 50 g/L,preferably not less than 75 g/L and is normally not more than 500 g/L,preferably not more than 300 g/L. If the amount of the oxidizing agentis too small, the oxidizing power is so low that the reaction may not bestopped. On the other hand, if the amount is too large, economy may below. In addition, the concentration of the acid such as hydrochloricacid and sulfuric acid used together with the oxidizing agent isnormally not less than 10 g/L, preferably not less than 15 g/L and isnormally not more than 500 g/L, preferably not more than 300 g/L. If theconcentration of the acid is too low, it may be difficult to dissolvethe substituent metal layer. On the other hand, if the concentration istoo high, the solution may erode other members than the aluminum oraluminum alloy basis material. Incidentally, the acid used here ispreferably a non-oxidizing acid, but it may be an oxidizing acid such asnitric acid or may be a mixture of an oxidizing acid and a non-oxidizingacid.

In the dissolving treatment as above-described, the treatment time isnot particularly limited; for example, the dissolving treatment can becompleted in a time of 5 to 300 seconds. The dissolving treatment may beconducted at a temperature of, for example, 10 to 40° C. Besides, duringthe dissolving treatment, the workpiece to be plated may be keptstationary or kept swinging, and the liquid may be agitated.

In forming a plating film after the surface treatment is conducted usingthe removing solution based on the present invention, the workpieceprovided with the substituent metal film on the aluminum or aluminumalloy surface as above-mentioned may be subjected directly to a platingtreatment or may be subjected to the plating treatment after thesubstituent metal layer is removed. In the latter case, since the oxidefilm on the aluminum or aluminum alloy surface has been removedcompletely, the subsequent electroless nickel plating, for example,results in that the aluminum of the basis material is replaced directlyby nickel. In addition, after the substituent metal film is removed, theworkpiece surface may be activated by a zinc substitution treatment, apalladium treatment or the like, before subjected to the platingtreatment. Such an activating treatment is preferably conductedparticularly by a zinc substitution treatment, more particularly analkaline zinc substitution treatment to thereby form a zinc coating filmon the aluminum or aluminum alloy surface, from the viewpoint ofenhancing the adhesion of the plating film.

Here, the zinc substitution treatment refers specifically to a treatmentof causing substitutive deposition of zinc by use of a solutioncontaining a zinc salt. The alkaline zinc substitution treatment is atreatment conducted using an alkaline zincate solution. Besides, anacidic zinc substitution treatment is a treatment of causingsubstitutive deposition of zinc by use of an acidic zinc salt-containingsolution. These treatments can be carried out by a known method.Further, the palladium treatment is a treatment of causing substitutivedeposition of palladium by use of a solution containing a palladiumsalt, and can be carried out by a known method.

The formation of the zinc coating film as above-mentioned is conducted,particularly in the field of semiconductor devices, as a pre-treatmentin the case of forming bumps by activating the surfaces of aluminum thinfilm electrodes patterned on a wafer and then conducting nickel plating,from the viewpoint of stable formation of the bumps. The zincsubstitution treatment conducted in such a case is a treatment methodwhich may lead to erosion of the aluminum or aluminum alloy basismaterial. However, the use of the removing solution based on the presentinvention ensures that the erosion of the aluminum thin film electrodesis restrained as securely as possible, so that even if the basissubstrate is a little eroded by the zinc substitution treatment, thealuminum thin film electrodes will be left more assuredly upon the zincsubstitution treatment.

The plating method for conducting the plating treatment after thesurface of a workpiece is treated by the surface treatment method basedon the present invention is not particularly limited, and may be anelectroplating method or an electroless plating method.

The electroless plating method is lower in energy than theelectroplating method, and, in carrying out the electroless platingmethod, a pre-treatment is particularly important for forming a platinglayer without defects.

According to the present invention, the impurities such as aluminumoxide film are completely removed, so that even the electroless platingmethod promises the formation of a plating layer with good adhesion.

Incidentally, when the electroplating method is adopted, wiring isneeded, which leads to the problems that it takes long to assemble theequipment, it may be impossible to enhance the density of the plating,and it may be difficult to form a uniform plating film due to generationof noises. These problems can be solved by use of the electrolessplating method.

The kind of the plating metal is appropriately selected according to theuse thereof. Examples of the plating metal include Cu, Ni, and Au, andthe plating layer may be composed of two or more layers of these platingmetals.

EXAMPLES

Now, Examples and Comparative Examples will be shown below so as todescribed the present invention more specifically, but the invention isnot limited to the following examples.

Examples 1 to 3, Comparative Example 1

A workpiece to be plated composed of a silicon plate coated with a 5μm-thick aluminum layer by a sputtering method was immersed in each ofremoving solutions prepared in compositions shown in Table 1, at 50° C.for 60 seconds. The removing solutions each had a pH adjusted to 12.4.Thereafter, the workpiece was immersed in a 500 ml/L aqueous nitric acidsolution at 21° C. for 1 minute, to dissolve away the metallic filmformed on the aluminum layer of the workpiece by immersion in theremoving solution. Further, an alkaline zinc substitution treatment wasconducted by immersing the workpiece in an alkaline zincate solution,then the workpiece was plated with nickel in a thickness of 0.5 μm by anelectroless plating method, and was further plated with gold in athickness of 0.05 μm by a substitutive plating method.

The plated workpieces thus obtained were each subjected to observationof the appearance, and the state of the plating film was evaluated. Inthis case, the electroless nickel plating film was formed in a smallthickness, and was coated further with the gold plating film. Thisensures that in the case where the oxide film is left unremoved,deposition of nickel (and gold) does not occur, and the plating-lackingarea is left as a hole (white in color) in the plating film. In view ofthis, the plating film-lacking state (oxide film remaining condition)was evaluated through comparison with gold color. The results arecollectively shown in Table 1 below.

TABLE 1 Comparative Concentration of ingredient Example Example in 1 Lof water 1 2 3 1 Removing Alkali (base) NaOH NaOH NaOH NaoH solutionSolubilizing agent EDTA•2Na (g/L) 10 10 10 10 Metallic salt (MetalMnSO₄•5H₂O 1 concentration · g/L) ZnSO₄•7H₂O 2 CuSO₄•5H₂O 0.05 pH 12.412.4 12.4 12.4 Appearance good good good bad Microscopic observation ofmetal (×500) no no no many hole hole hole holes

Examples 4 to 6, Comparative Example 2

A workpiece to be plated composed of a silicon plate coated with a 1μm-thick Al—Si (Si content: 0.5 wt %) layer by a sputtering method wasimmersed in each of removing solutions prepared in compositions shown inTable 2, at 50° C. for 60 seconds. The removing solutions each had a pHadjusted to 12.4. Thereafter, the workpiece was immersed in a 500 ml/Laqueous nitric acid solution at 21° C. for 1 minute, to dissolve awaythe metallic film formed on the Al—Si layer of the workpiece byimmersion in the removing solution. Further, an alkaline zincsubstitution treatment was conducted by immersing the workpiece in analkaline zincate solution, then the workpiece was plated with nickel ina thickness of 0.5 μm by an electroless plating method, and was furtherplated with gold in a thickness of 0.05 μm by a substitutive platingmethod. The plated workpieces thus obtained were each subjected toobservation of the appearance, and the state of the plating film wasevaluated. The results are collectively shown in Table 2 below.

TABLE 2 Comparative Concentration of ingredient Example Example in 1 Lof water 4 5 6 2 Removing Alkali (base) NaOH NaOH NaOH NaOH solutionSolubilizing agent EDTA•2Na (g/L) 10 10 10 10 Surfactant PEG-1000 (g/L)1 1 1 1 Metallic salt (Metal MnSO₄•5H₂O 1 concentration · g/L)ZnSO₄•7H₂O 2 CuSO₄•5H₂O 0.05 pH 12.4 12.4 12.4 12.4 Appearance good goodgood bad Microscopic observation of metal (×500) no no no many hole holehole holes Note) PEG: polyethylene glycol

Examples 7 to 9, Comparative Example 3

A workpiece to be plated composed of a silicon plate coated with a 5μm-thick aluminum layer by a sputtering method was immersed in each ofremoving solutions prepared in compositions shown in Table 3, at 50° C.for 60 seconds. The removing solutions each had a pH adjusted to 12.4.Thereafter, the workpiece was immersed in a 500 ml/L aqueous nitric acidsolution at 21° C. for 1 minute, to dissolve away the metallic filmformed on the aluminum layer of the workpiece by immersion in theremoving solution. Further, an alkaline zinc substitution treatment wasconducted by immersing the workpiece in an alkaline zincate solution,then the workpiece was plated with nickel in a thickness of 0.5 μm by anelectroless plating method, and was further plated with gold in athickness of 0.05 μm by a substitutive plating method.

The plated workpieces thus obtained were each subjected to observationof the appearance, and the state of the plating film was evaluated. Inthis case, the electroless nickel plating film was formed in a smallthickness, and was coated further with the gold plating film. Thisensures that in the case where the oxide film is left unremoved,deposition of nickel (and gold) does not occur, and the plating-lackingarea is left as a hole (white in color) in the plating film. In view ofthis, the plating film-lacking state (oxide film remaining condition)was evaluated through comparison with gold color. The results arecollectively shown in Table 3 below.

TABLE 3 Comparative Concentration of ingredient Example Example in 1 Lof water 7 8 9 3 Removing Alkali (base) TMAH TMAH TMAH TMAH solutionSolubilizing agent EDTA•2Na (g/L) 10 10 10 10 Metallic salt (MetalMnSO₄•5H₂O 1 concentration · g/L) ZnSO₄•7H₂O 2 CuSO₄•5H₂O 0.05 pH 12.412.4 12.4 12.4 Appearance good good good bad Microscopic observation ofmetal (×500) no no no many hole hole hole holes

Examples 10 to 12, Comparative Example 4

A workpiece to be plated composed of a silicon plate coated with a 1μm-thick Al—Si (Si content: 0.5 wt %) layer by a sputtering method wasimmersed in each of removing solutions prepared in compositions shown inTable 4, at 50° C. for 60 seconds. The removing solutions each had a pHadjusted to 12.4. Thereafter, the workpiece was immersed in a 500 ml/Laqueous nitric acid solution at 21° C. for 1 minute, to dissolve awaythe metallic film formed on the Al—Si layer of the workpiece byimmersion in the removing solution. Further, an alkaline zincsubstitution treatment was conducted by immersing the workpiece in analkaline zincate solution, then the workpiece was plated with nickel ina thickness of 0.5 μm by an electroless plating method, and was furtherplated with gold in a thickness of 0.05 μm by a substitutive platingmethod. The plated workpieces thus obtained were each subjected toobservation of the appearance, and the state of the plating film wasevaluated. The results are collectively shown in Table 4 below.

TABLE 4 Comparative Concentration of ingredient Example Example in 1 Lof water 10 11 12 4 Removing Alkali (base) TMAH TMAH TMAH TMAH solutionSolubilizing agent EDTA•2Na (g/L) 10 10 10 10 Surfactant PEG-1000 (g/L)1 1 1 1 Metallic salt (Metal MnSO₄•5H₂O 1 concentration · g/L)ZnSO₄•7H₂O 2 CuSO₄•5H₂O 0.05 pH 12.4 12.4 12.4 12.4 Appearance good goodgood bad Microscopic observation of metal (×500) no no no many hole holehole holes Note) PEG: polyethylene glycol

1. A solution for removing an aluminum oxide film from an aluminum oraluminum alloy surface, comprising a salt or oxide of a metal capable ofsubstituting aluminum, a solubilizing agent for ions of the metal, andan alkali and having a pH of 10 to 13.5.
 2. The solution for removingthe aluminum oxide film of claim 1, further comprising a surfactant. 3.A method for surface treatment of aluminum or the aluminum alloy,comprising the steps of: immersing a workpiece having aluminum or thealuminum alloy at least at the surface in the solution of claim 1, andforming a substituent metal layer of the metal capable of substitutingaluminum and contained in the solution on the aluminum or aluminum alloysurface while removing the aluminum oxide film from the aluminum or thealuminum alloy surface.
 4. The method for surface treatment of aluminumor the aluminum alloy of claim 3, wherein the workpiece is a body havingthe aluminum or aluminum alloy film formed on a surface of anon-aluminum material.
 5. The method for surface treatment of aluminumor the aluminum alloy of claim 3, wherein after the substituent metallayer is formed, a plating layer is formed.
 6. The method for surfacetreatment of aluminum or the aluminum alloy of claim 3, wherein afterthe substituent metal layer is formed, the substituent metal layer isremoved by an acidic liquid having an oxidizing behavior.
 7. The methodfor surface treatment of aluminum or the aluminum alloy of claim 6,wherein after the substituent metal layer is removed by the acidicliquid having the oxidizing behavior, the aluminum or aluminum alloy issubjected to a zinc substitution treatment or a palladium treatment andis then plated.
 8. The method for surface treatment of aluminum or thealuminum alloy of claim 6, wherein after the substituent metal layer isremoved by the acidic liquid having the oxidizing behavior, the aluminumor aluminum alloy is subjected directly to plating.